Zirconia (ZrO.sub.2) exhibits three well defined polymorphs: The monoclinic phase (stable up to 1170.degree. C.), the tetragonal phase (stable from 1170.degree. C. to 2370.degree. C.), and the cubic phase (stable from 2370.degree. C. to melting point of 2680.degree. C.). Another polymorph, the orthorhombic phase also exists and is usually found at high pressures. The tetragonal phase can be retained at room temperature. The interest in zirconia stems from the fact that under service conditions, the tetragonal zirconia can transform to monoclinic phase by stress induced transformation thereby arresting crack propagation. This is usually called transformation toughening. Detailed description of importance and significance of zirconia ceramics can be found in Advances in Ceramics Science and Technology of Zirconia, vol. 3 (1981), (eds. A. H. Heuer and L. W. Hobbs) and vol. 12 (1984) (eds. N. Claussen, M. Ruhle and A. Heuer), Am. Ceram. Soc., Inc., Columbus, Ohio; R. C. Garvie, "Zirconium Dioxide and some of its Binary Systems", in High Temperature Oxides, Vol. II, ed. A. M. Alper, pp. 118-166; R. Stevens, "An Introduction to Zirconia", Magnesium Elecktron Ltd., Flemington, NJ.
By adding stabilizing oxides such as yttria (Y.sub.2 O.sub.3), calcia (CaO), magnesia (MgO), other rare earth oxides to zirconia either partially stabilized zirconia (which is a mixture of cubic, tetragonal and/or monoclinic), or fully tetragonal zirconia or fully stabilized zirconia (which is mostly cubic phase) can be obtained depending upon the amount of additive and method of preparation. Ideally the stabilizing additive should be distributed uniformly and this is accomplished by appropriate powder preparation. Starting powder preparation can be carried out by dry mixing or wet mixing. Wet mixing is usually done either by sol-gel techniques, or by co-precipitation of hydroxides, or by hydrolysis of chlorides or organometallics, etc. . . . There are partially or fully stabilized zirconia powders available commercially, for example, from Toyo Soda Manufacturing Company, Ltd. Tokyo, Japan and Magnesium Elektron Inc., NJ.
The conventional methods for fabricating zirconia ceramic parts are pressureless sintering, hot-isostatic pressing, and hot pressing.
Pressureless sintering is accompanied by non-uniform shrinkage which makes it impossible to meet close tolerances without subtantial machining.
Hot-isostatic pressing requires the use of a cladding to separate the pressurizing gas from pores of the powder compact or a precalcination to create an impervious structure and produces product where flaws are generally reduced in size but still retained.
Hot pressing normally is utilized to press into pancake or billet shapes which are machined into final shape.